Go to any pro tennis tournament and you will see players sporting a variety of braces, tape, bandages, and more tape. The Philadelphia Advanta Championships was no exception. Amelie Mauresmo sported heavy tape around her left upper thigh. Teryn Ashley wore an ankle brace like Andy Roddick's; Ashley's was on her left ankle where Andy Roddick wears them on both ankles. Vera Zvonareva, the eventual finalist, had her right wrist taped throughout the week. Venus Williams routinely plays with elaborate taping on both wrists, about the width of two extra-wide wristbands. Meilen Tu had a V-shaped collection of patches on the front of her knee. Rossana de los Rios played with a strap around her entire calf just below the knee.

Billie Jean King says there was maybe one day in her entire career when she didn't play with some niggling injury.

The watcher at home or at courtside, however, wants to know: what do the various braces mean? How serious an injury is it if the player can take the court? Are all those apparently effortless shots actually agony for the player to produce?

Dr. David Rubenstein, the physician for the Philadelphia tournament, explains.

"Each body part can have some type of brace or support," he says, adding that for a tournament like Philadelphia he brings 20 different types of braces with him. Rubenstein divides these into three categories. First is preventative. High school linemen, for example, who have never had an injury, might wear a brace to ensure that they stay injury-free. Professional tennis players do not wear these, in part because any brace impedes your freedom of movement, even if only by a little bit.

The second category is rehabilitative. These tend to be very large braces designed to allow recovery from an injury. They protect the injured body part so it can heal, hence the size. Often, these braces severely restrict motion. The massive brace Kim Clijsters was wearing on her left wrist at courtside during then-fiance Lleyton Hewitt's matches for much of this year is an example. No one plays in one of these.

The third category, which is the one we see everywhere on court, is functional. These are designed for situations where someone has an injury but is trying to perform. They are often expensive, in part because if a player is going to be wearing them for months they must not only be durable but decorative.

The first type of functional supports are wraps and sleeves. "None of them really provide any stability," says Rubenstein, "but they make you feel good." For one thing, such wraps -- like the notorious one-sleeve shirt that Martina Hingis sported during the Australian Open one year -- provides compression. For another thing, they do provide some sense of stability if they are wrapped around a joint.

The reason, says Rubenstein, is known as "proprioception," the ability to know where you are in space. These wraps, including the ubiquitous tape, provide additional feedback to the nerves because of the way they move or resist against the skin, so the athlete can feel better how she is moving. She is therefore less likely to make a physical error that might exacerbate the injury. This, he says, is why so many tape their ankles.

Ashley's ankle brace is known as an Aircast, and Rubenstein says its usefulness is about 50/50 stability and proprioception.

De los Rios's knee band, on the other hand, is known as "Cho pat," and it stops the tendon from vibrating, the same principle as the one behind the many braces sold for tennis elbow. In the case of the knee band, the tendon in question is the patellar tendon -- the one that attaches to the kneecap -- and stopping it from vibrating mediates tendonitis (inflammation of the tendon). If you think of the tendon as a guitar string that's vibrating, says Rubenstein, and the brace as the functional equivalent of putting your finger on it, the principle is simple to understand. Similarly, wrist braces decrease excessive vibration and for much the same reason.

Mauresmo's (and Elena Dementieva's) thigh tape -- or thermoskin, depending what's used -- again compresses the muscle, so it feels better, and also helps heat the muscle so it's more pliable, making muscle pulls less likely.

The popularity of tape is simple: "A good trainer can apply tape so it's much less cumbersome than any brace." Since the WTA tour has some of the best trainers, players are more likely to get themselves taped before every match than to carry braces with them. But, he warns, "Never try to tape yourself. It requires talent and skill."

Rubeinstein warns, however, that it's difficult to tell just from looking how serious an injury really is. There wasn't much doubt about Cara Black, who twisted her ankle and had to pull out of singles qualifying (though she made it to the doubles semifinal). But, "A lot of players, you successfully treat the injury and they get better, but a year later with no injury they still have the tape. You can't tell how many braces are ritualistic and how many are support." But, he says, "If you're out on court, how bad can it be?"